Cement raw mix and clinker grinding mill control system



Dec. 5, 1961 J. H. HERZ 3,011,726

CEMENT RAW MIX AND CLINKER GRINDING MILL CONTROL SYSTEM Filed Oct. 21, 1959 3 Sheets-Sheet l (ESE/0H I]. H522 .Zv EA/TOE Warm flrTOBA/ Y Dec. 5, 1961 J. H. HERZ 3,011,726

CEMENT RAW MIX AND CLINKER GRINDING MILL CONTROL SYSTEM Filed on. 21, 1959 3 Sheets-Sheet 2 Powse 7/ 38 Supp; y

50 6/ 1 6, 2. 2,055 1!. filsez WM NM Arr-02MB ys Dec. 5, 1961 J. H. HERZ 3,011,726

CEMENT RAW MIX AND CLINKER GRINDING MILL CONTROL SYSTEM Filed Oct. 21, 1959 3 Sheets-Sheet 3 III B C M 37 3a F Com/E2752 44 Coureause 7055 H H522 fvVE/v 70E ,4 TfOENE Y5 3,011,726 Patented Dec. 5, 1961 3,011,726 I CEMENT RAW MIX AND CLINKER GRlNDiNG MILL CONTROL SYSTEM Joseph H. Her-z, Redlands, Califi, assignor to California Portland Cement Company, Los Angeles, Calif., a corporation of California Filed Oct. 21, 1959, Ser. No. 847,766 9 Claims. (Cl. 24130) This invention relates generally to the control of feeding of raw grindable material to a grinding mill, and more particularly has to do with relating the mill feed to the amount of oversize material being returned to the mill from a separator or separators acting to size-classify the mill discharge, all in a novel manner enabling very close control of the complete grinding and classifying operations.

In the past is has been found necessary'to make provision for varying the rate of raw unground material being fed to a grinding mill, due to the variance in the degree of hardness and in the size of raw material and manufactured materials such as gypsum, shale, limestone, and the like. In many cases, an operator has been used for regulating the feed of unground material to the mill, the operator being liable to underfeed or overfeed the mill after an undesirable time lag required for visual observation of changes in the separator operation, so that the mill does not achieve continuous optimum production. In those instances where the operator has been replaced by feed control devices, these have not, to my lmowledge, been able to achieve optimum mill production as related to operation of a classifier or separator, primarily because of inaccuracies and time lags in controlling the mill feed in response to changes in the separator operating conditions.

Accordingly, it is a major object of the invention to overcome these prior existing disadvantages through continuous and incremental control of the mill feed in response to incremental changes in the actual work performed by the mill, measuring as power consumed by the separator or separators in which the mill discharge is classified. To illustrate, the power consumption of the separator or separators depends upon the amount of material fed to it and classified as finished product. If the material fed to the mill and the mill operating conditions remain uniform the material will be reduced at a uniform rate in the mill and will be delivered at a uniform rate from the mill to the separator, so that the finished product and the separator tailings will be produced at a uniform rate.

However, should the hardness or size of the raw grindable material or the mill operating conditions be subject to change, the rate of reduction in size of the material in the mill will be increasingly changed. Furthermore, the percentage or the amount of tailings coming from the separator and returned to the mill will also vary, so that unless the rate of feed of unground material to the mill is adjusted the tailings returned to the mill will, for example, build up. As as result the grinding efiicicncy of the mill will be further reduced and will approach the point where the mill will over-fill and overflow at the feed end. Also, the build up of tailings being recycled through the mill and the separator will increase the power consumption of the separator and ultimately cause stoppage. Other unfortunate results may also be caused by such build up of tailings, as for example overloading of the materials transfer equipment such as elevators, screw conveyors and the like.

In accordance with the present invention, and referring first to the method concept, there is produced a continuous signal that is incrementally and proportionally variable in response to incremental changes power consumption of the motor driven separators, that signal being used to control the rate of feed of raw grindable material to the mill such that an incremental increase in the power consumption of the separator results in an immediate and proportional incremental decrease in the feed rate to the mill. As a preferred method of producing such a signal, a thermocouple is heatedin proportion to the power consumption of the motor driven separator, the thermocouple signal representing an accurate and continuous measurement of the work performance of the separator in classifying the discharge from the mill. Such a thermocouple signal is typically produced by means of a thermal converter or thermal watt meter of the type described at page 414 of Plant Engineering Handbook, by William Stanier, published in 1950 by McGraw-Hill Book Company, and is typically usable to control the electrical resistance in the field circuit of a direct current motor driving mechanism feeding grindablematerial to the mill, thereby to control the feed rate in accordance with the principles of the invention.

An equally important aspect of the invention-concerns the utilization of two or more separators to which the mill discharge is fed after being split or divided into parallel streams, the division or splitting of the mill discharge being controlled by means of continuous signals that are incrementally and proportionally variable in response to incremental changes in the power consumption of the motor driven separators. As will appear, the signals representing such power consumption of the separators are balanced for controlling division of the mill discharge so that the separators consume power in predetermined relationship, as for example, equal power consumption by equal sized separators. e signals are also combined or added for controlling the feed rate of raw unground material to the mill so as to prevent overloading and overfilling of. the mill.

These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following detailed description of the drawings, in which:

FlG. l is an elevation showing process equipment including a mill, a separator, materials handling equipment, and the particular control loop of the present invention;

FIG. 2 is a plan view showing the mill havingits discharge split and fed to a pair of separators, together with the control equipment for controlling feed to the mill and splitting of the mill discharge;

FIG. 3 is a schematic diagram of the control loop for a mill with one separator and a number of separate drives for separate mill feeders; and

FIG. 4 is a schematic circuit diagram showing the manner in which a direct current mill feeder motor is speed controlled.

Referring first to FIG. 1, a rotary ball milljltl of usual mechanical construction is shown mounted on bearings 11 carried by a concrete foundation 12, the mill having an input end 13 and an output end 14. A motor 15 drives the mill in rotation through the speed reduction gearing including small and large spur gears 16 and 17. Raw unground material, such as limestone, shale, gypsum, clinker or the like is typically fed to the mill from a storage bin 18 by apparatus including an endless belt feeder or conveyor 19 receiving discharge from the bin through a chute 20, the material falling off the feeder into a chute 21 delivering into the mill inlet. The feeder is shown as being driven by a direct current motor 22 and belt 23.

Ground material leaving the mill it) falls through a chute 24 and is conveyed at 25 to an elevator 26 driven by motor 27. The elevated material is conveyed by screw conveyor 28 to the separator 29 into which the material falls for separation into a product stream leaving the separator at 30 and into a tailings stream leavingthe mal converter or watt meter indicated at 38.

tom differential between the junctions.

separator at 31. Such a separator is driven by an induction motor 32, and is described in US. Patent 1,615,558 granted to Thomas J. Sturtevant on January 25, 1927. Thefinished product stream may be conducted away from the separator by screw conveyor 33, and the tailings or oversize material rejected by the separator returns to the mill inlet through chute 31, for further grinding.

In accordance with the invention, the power consump tion of the separator motor 32 is measured by means of a potential transformer indicated generally at 34 and a current transformer indicated generally at 35, the motor leads from which these transformers are energized being shown at 36. The transformers are shown as being electrically connected through cables 36 and 37 with a ther- Such a converter is known, as previously mentioned, and the circuitry shown is merely illustrative of the more detailed circuitry of the actual device. However, in all cases it will include a resistance type heater or heaters 39 to which the secondaries of the transformers 34 and 35 are electrically connected and a thermocouple or thermocouples 4t having a hot junction 41 in position to be heated by the heater 39, as well as an appropriate cool junction 42 the temperature of which is regulated to maintain a tempera- The output of the converter, i.e., the millivolt direct current signal of the thermocouple appearing at leads 43, is an accurate measure of the true power consumption of the motor 32. Also, the true power consumption of the motor 32 is proportional to the amount of work performed by the separator to separate the product size stream at 30 and the tailings stream at 31 so that should the tailings stream increase, the millivolt signal output of the thermal converter will also change.

The converter signal output is then fed to a process controller indicated generally at 44 for modulating the power output of the controller fed at 45 to actuator 46,

the power input to the controller being indicated at 47. Such controllers are by themselves well known in the art and will not be described herein. The actuator 46 may take the form of a speed modulator for the direct current motor 22, in which case reference to FIG. 4 will show that the modulator controls the resistance 48 in series with the armature field coil 49 of the motor. Specifically, the actuator 50 controls the tap or wiper arm 51 movable along the resistance or rheostat 48 through which power is delivered to the field coil 49, such that changes in the speed of the motor 22 is incrementally proportional to changes in the signal output of the thermal converter. A.C. power is delivered at 147 to the modulator 46, and a suitable rectifier 52 is inserted in the direct current motor circuit as shown in H6. 4. The modulator circuitry just described is merely illustrative, and other equivalent forms thereof may of course be utilized provided the manner of'control of raw materials feed as contemplated by the invention is not changed.

Referring now to FIG. 2, the various elements which are the same as those shown in FIG. 1, including the mill and its feeding apparatus, are given the same numbers. However, in this illustration the discharge leaving the mill through chute 55 or equivalent materials handling apparatus, is split into separate parallel streams indicated at 56, as by a pivoted vane type divider 57 in the chute 55. An arm 58 for controlling the vane angular disposition is controllable by the actuator 59.

The parallel discharge streams 56 flow through appropriate chutes 60 to elevators 61, and then to and through horizontal screw conveyors 62 in the direction of the arrows 63, the latter delivering the parallelstreams of material to the separators 64. Theseare of the same type as previously described at 29, being driven by motors indicated at 65. The tailings from the separators are returned to the mill inlet 13 through chutes 31 also as previously described, while the product material is removed through appropriate materials handling equipment, not

shown.

Each of the separators 64 has associated therewith a thermal converter or watt meter indicated at 33 to which the voltage and current transformers 34 and 35 associatedzwith the separator drive motors 65 are electrically connected as through cables 36. and 37. The thermal converters produce signals which are accurately and incrementally proportional to tthe power consumed by th individual separators, and these signals are used in accordance with the invention to control the feed of raw unground material to the mill and also the division of discharge material from the mill into the separate parallel streams 56, so as to cause the separators to consume power in predetermined relationship, for example equal power consumption. Referring first to the. control of the mill feed, the opposite polarity signals from the thermal converters are fed through lines 70 and to the process controller 44 wherein these signals are added for modulating the power supplied at 72 to the controller 44. The modulated power output 45 of the controller is fed in turn to the speed modulator 46 for controlling the speed of the feeder motor 22 in such relation that an incremental increase in the power consumption of both separators results in a proportional incremental decrease of raw unground material fed to the mill. In both FIGS. 1 and 2 and in FIG. 4 the line through which modulated power is delivered to the motor 22 from the modulator 46 is shown at 75.

Manual control of the slide wire resistor 48 shown in F168. 1, 2 and 4 may be substituted for the actuator 5% as indicated by the manual control handle 76 in FIGS. 1 and 2. in such event, the operator carefully watches the indicator or dial 77 on the controller :4, which visibly indicates changes in the signal output of the thermal converter or converters 38. Thus, the operator uses the signal output of the thermal converter or converters 38 to control accurately the rate of feed of raw g'rindable material to the mill in such a way that an incremental increase in the power consumption of the separator would respond in a proportional incremental decrease in the feed rate in the mill.

Referring now to HG. 3, the control apparatus shown in schematic form consists of the same basic components; however, four different motors 8%) are shown for driving four different feeders adapted to feed dilferent materials simultaneously to the mill. Thus, such materials as Shale, gypsum, limestone and clinker may be simultaneously fed to the mill.

The speed modulator 46 in this case controls the speed of motor generator 81, as for example by controlling the amount of resistance in the field windings of a direct current motor 82 driving the alternator 83. The alternator in turn supplies current to the feeder motors 89 through line 8 and rectifiers 85. Thus, the speeds of the slave motors 8% driving the feeders are controlled to remain in listed proportion to one another by means of the speed modulator 46 controlling the motor generator unit 81.

If desired, the speeds of the individual motors 3% can be individually and relatively adjusted as by adding or subtracting resistance in the field coil circuits of the motors. Such a resistance adjusting device is shown schematically at to include resistance selectors 86 coupled to the motors 39 through the lines 87. Such feed control will increase and decrease the speeds of the battery of feeders to match the grinding capacity of the mill, without upsetting the selected proportional feeds of the diiferent materials.

In case an electronic power modulator is used to provide the variable speed motors at with power the speed modulator will be connected to a series of 10,900 ohm slidewires or resistances in the field circuits of the motors 80, adjustment of which will increase or decrease the speeds of the feeders. Such slidewires would correspond to that shown in FIG. 4, with one slidewire for each motor 80.

I claim:

1. In the continuous process wherein dry raw grindable material is fed to a grinding mill, the dry mill discharge being subsequently sizeclassified by motor driven separators, the power consumption of the separators being subject to predetermined change whenever the separator separates out increasing amounts of oversize material which is returned to the mill, the steps that includes variably dividing the mill discharge into streams simultaneously fed to the separators, producing continuous direct current signals that are incrementally and proportionally variable in response to incremental changes in the power consumption of the motor driven separators, and using said signals to control the rate of feed of raw grindable material to the mill thereby to incrementally decrease the feed rate in response to an incremental predetermined change in the combined separator power consumption.

2. The invention as defined in claim 1 including the step of adding said signals for controlling said feed rate.

3. The invention as defined in claim 1 including using said signals to control said division of the mill discharge thereby to maintain substantially constant predetermined power consumption at said separators.

4. The invention as defined in claim 3 including the step of balancing such signals for controlling division of the mill discharge such that the separators consume power in predetermined relationship.

5. Apparatus for controlling the feed of raw grindable material to a grinding mill in a system including separators continuously size-classifying ground material discharged from the mill and individual motors driving said separators, the power consumption of the separators being adapted to increase whenever the separators separate out increasing amounts of oversize material which is returned to the mill, said apparatus comprising means producing continuous signals that are incrementally and proportionally variable in response to incremental changes in the power consumption of the respective motor driven separators, circuit means for adding said signals to produce a resultant signal, and means controlled by said resultant signal and controlling the rate of feed of raw grindable material to the mill such that an incremental increase in the power consumption of separator power consumption results in a proportional incremental decrease in said raw material feed rate.

6. The invention as defined in claim 5 in which said signal producing means includes different thermocouples having hot junctions adapted to be heated in proportion to the power consumption of the different individual motors driving the separators.

7. The invention as defined in claim 5 including a movable divider operating to divide the mill discharge into separate streams simultaneously fed to the different separators, and means balancing said signals for controlling said divider to separate the discharge into streams proportioned such that the separators consume power in predetermined relationships.

8. The combination, comprising a grinding mill and a feeder operating to feed raw grindable material to the mill, separators continuously size-classifying streams of ground material discharged from the mill, individual induction motors driving the separators so that the power consumption of the motors is adapted to increase whenever the separators separate out increasing amounts of oversize ground material, means continuously returning said oversize material to the mill for further grinding, means sensing the current and voltage inputs to said separator motors and utilizing thermocouples to produce small continuous direct current electrical signals that are incrementally and proportionally variable in response to incremental changes in the power consumption of the respective separator motors, electrical circuit means for adding said signals to produce a resultant signal, and means controlled by said resultant signals and controlling the speed of the feeder such that an incremental increase in the power consumption of the separators results in a proportional decrease in the raw material feed rate.

9. The invention as defined in claim 8 including a movable divider operating to divide the mill discharge into separate streams simultaneously fed to the different separators, and means balancing said signals for controlling said divider to separate the discharge into streams proportioned such that the separators consume power in predetermined relationships.

References Cited in the file of this patent UNITED STATES PATENTS 1,413,934 Ramsey Apr. 25, 1922 2,240,822 Adams May 6, 1941 2,491,466 Adams Dec. 20, 1949 2,499,347 Adams Mar. 7, 1950 

